Micromachining may performed by directing beams, such as ion beams, electron beams, laser beams, molecular beams, cluster beams, or atom beam toward a work piece. For example, focused ion beam systems are used in forming, shaping or altering microscopic structures, such as electronic circuit components and micro-electromechanical system (MEMS) structures. A focused ion beam can be focused to a very small spot on the work piece and then scanned over the surface in a desired pattern to remove material.
As an ion impinges on the work piece surface, its momentum is transferred resulting in the removal of one or more surface atoms by a process called “sputtering.” By selecting a pattern of a given overall shape, for example a horizontal raster pattern, a correspondingly shaped area of surface material can be removed. Often several successive layers of a semiconductor device are removed in a given area in order to reach and possibly sever an underlying layer. Because an ion beam can be finely focused, it can create a fine structure.
The above described physical sputtering process can be enhanced by the introduction of an etch precursor gas. The gas is adsorbed on the surface of the work piece prior to arrival of the ions and the gas reacts chemically with the surface material in the presence of the ion beam to facilitate sputtering and reduce redeposition of the sputtered material. The ion beam may induce decomposition of the precursor gas into reaction products, some of which react with the work piece material. A precursor gas chemically reacts to form volatile compounds, resulting in a substantial increase in sputtering rates. For instance, an enhancement factor in sputtering of silicon with a chlorine precursor gas has been reported of approximately fourteen, i.e., the gas-enhanced sputtering occurs approximately fourteen times as fast as sputtering without the introduction of the gas. Gas-enhanced sputtering is also less subject to redeposition of sputtered material. The enhancement factor for metal surfaces, such as conductive layers in a semiconductor device, may be even greater.
Electron beams typically lack the momentum to sputter material, but can be used to initiate a reaction between the precursor gas and the work piece, and thereby etch a work piece surface. See, for example, U.S. Pat. No. 6,753,538 for “Electron Beam Processing” to Musil et al. which is assigned to the present applicant. Other types of beams can also be used to initiate the reaction between the precursor gas and the work piece to etch the surface, including laser beams, as described for example, in U.S. Pat. No. 5,874,011 to Ehrlich for “Laser-induced Etching of Multilayer Materials” and cluster beams, as described in U.S. Pat. No. 8,835,880 to Chandler, et al. for “Charged Particle-beam Processing Using a Cluster Source,” which is assigned to the present applicant. Additional types of beams, such as beams of neutral atom or molecules can also be used to initiate reactions between the precursor gas and the surface. Beams can be focused to a fine point, or can be broad to process larger areas.
To be useful as an etch precursor for beam processing, the gas molecules should have very specific properties: they need to stick to the surface for a sufficient time to react with the beam, but they must not form a thick layer that shields the surface from the beam. The gas should not react spontaneously with the work piece surface material in the absence of the beam. The precursor dissociation products should form a volatile compound with the work piece material. Etch precursors are typically specific to a particular work piece materials, and so can be used for selectively etching certain materials. That is, the beam-induced etching preferentially etching some materials over other materials, facilitating, for example, etches away one layer without destroying an underlying layer.
Halogen-containing precursor gases are often used for beam-induced etching because the reaction products tend to be gaseous and can be removed from a sample vacuum chamber by the vacuum pump. Elemental halogens, such as chlorine or iodine, are used as precursor gases but have disadvantages. The introduction of chlorine or fluorine gas creates a safety hazard because of the combined toxicity and high vapor pressure of the gas. Furthermore, chlorine gas is often chemically reactive with the entire work piece surface and may undesirably attack areas adjacent to the ion beam position, resulting in a lack of contrast between machined and non-machined areas. Moreover, chlorine causes corrosion of typical materials used in construction of the chamber and components within as well. Chlorine and bromine require handling of in halogen-compatible hardware, which is typically expensive. Handling tanks of toxic gas at high pressures is awkward even in a carefully monitored laboratory environment, requiring the use of chlorine leak sensors.
U.S. Pat. No. 5,188,705 to Swanson et al. for “Method of Semiconductor Device Manufacture” describes the use of iodine vapor as a precursor gas to replace chlorine and fluorine. Iodine has a long residence time within a charged particle beam system. This causes corrosion of components within the chamber, particularly if exposed to water vapor from air.
To avoid some of the disadvantages of elemental halogen gases, other halogen-containing precursor gases have been used as precursor gases. For example, WO00/022670 of Chandler, for “Integrated Circuit Rewiring Using Gas-Assisted FIB Etching,” describes the use of trifluoroacetamide and trifluoroacetic acid as precursor gases. These compounds have lower toxicity and more convenient material handling properties. Currently, no similar compounds are known as chlorine or bromine sources. As an example, trichloroacetic acid is not an effective etch precursor gas.
XeF2 has been used as a precursor gas for beam-induced etching as described, for example, in U.S. Pat. No. 6,753,538 for “Electron Beam Processing” to Musil et al. XeF2, however, spontaneously etches many materials, including silicon and TaN. XeF2 is highly corrosive and toxic, requiring special handling and safety procedures. XeF2 cannot be mixed with many common gases used for residual carbon removal and surface species control. Moreover, large quantities of XeF2 cause instability in some differentially pumped beam systems because of poor ion getter pumping of xenon.
One application for focused ion beam etching is the preparation of thin samples for viewing on a transmission electron microscopy (TEM). The beam is directed towards the work piece surface to form the thin sample, typically without using an etch-assisting gas. When the work piece is composed of a III-V semiconductor compound, the beam etches the group V elements (N, P, As . . . ) elements at a higher rate than it etches the group III elements (Ga, In), because the group V elements have greater volatility. In a work piece including Ga or in, for example, ion beam sputtering can lead to the formation of Ga droplets and In crystals by diffusion of excess Ga and In within the sample. FIG. 1A shows a work piece 104, in which etching has caused In crystals 102 to form. FIG. 1B shows a similar prior art work piece 208, in which a gallium droplet 210 has formed.
It would be useful to find a system for beam-induced etching that solves some of the problems described above.